Lead acid battery with improved performance

ABSTRACT

In a lead-acid battery with stress applied perpendicular to the plane of the electrodes, at least one of the elements chosen from among the positive electrode, the negative electrode and the electrolyte has been modified so that the quantity of sulfuric acid in the positive electrode and/or negative electrode represents at least 0.20 mole of H2SO4 per mole of active material in the charged state.

This application is a national phase of PCT/FR99/01022 which was filedon Apr. 29, 1999, and was not published in English.

TECHNICAL FIELD

The invention relates to lead-acid batteries having improved performancecompared with current lead-acid batteries.

Batteries of this type are particularly intended for the production ofhigh-performance batteries, for electric vehicles for example.

STATE OF THE ART

During the 1970's, research into high performance batteries intended forelectrical vehicles confirmed a known dilemma, but which particularlyconcerns lead-acid batteries: choosing between weight performance andendurance, improvement in one being achieved to the detriment of theother. At times priority was given to energy-to-weight ratio to such anextent that lifetimes fell to scarcely acceptable values. Thereforeresearch was conducted to arrive at a better compromise.

During a 5-hour discharge under constant current, the energy-to-weightratio of lead-acid batteries intended for electric vehicles ranges from30 to 40 Wh/kg. On board vehicles, this range of magnitude is reduced by20%. Therefore, at discharge times of 1 to 2 hours, the lead-acidbattery proves to give two times less performance than thenickel-cadmium battery, and three times less than the sodium-sulphurbattery.

It would therefore appear that the lead-acid battery ranks the lowestamong possible candidates for the market of electric vehicles. Yet, overand above its price, some prospects plead in its favour.

Its performance in terms of charge time should increase with the arrivalof specific vehicles that are lighter and more energy-saving. Itslifetime could be increased through intelligent management of itsenergy. Moreover, the lead-acid battery is probably, among thosebatteries competing for the new markets, the one which has the highestrelative margin for progress.

To improve the performance of lead-acid batteries, it is required toincrease the coefficient of use of the active materials of theelectrodes. Two pathways of research can be considered to achieve thisresult. One concerns the collection of charges on the side of the activeelectrode materials. The other concerns a better distribution of thereagents within the electrodes.

The active material of the electrodes is in fact only used very little,even during a so-called complete discharge, the percentage of convertedmaterial being in the region of 25 to 30%.

In respect of charge collection, the electron exchanges between theactive material and the outer circuit are ensured by lead alloyconductors in the form of grids or ribs. The size of these collectors isdetermined by the two following factors: the fact that they form themechanical support for the electrodes, and the need to resist corrosionphenomena to which they are subject inside the electrodes. Inconsequence, in current assembly technology, so-called single poleassembly, the weight of the collectors represents between 40 and 50% ofthe weight of the electrodes for electric vehicle applications.

Regarding the distribution of reagents inside the electrodes, this islimited by the electrode porosities which may be used, as will be seenbelow.

For over a century, the plates of lead-acid batteries have been madeusing the Fauré method, the so-called grid and added oxide method. Moreprecisely, a grid in lead alloy is lined with a paste made of leadoxide, sulphuric acid and water. The proportions of these variousconstituents were empirically determined having regard to theperformance of the battery. It soon became apparent that the increase inthe quantity of water, that is to say in resulting porosity, increasesinitial capacity to the detriment of lifetime. Values held to beacceptable for these two parameters limit variations in porosity to anarrow range, in the region of 10%.

Inside an electrode of a lead-acid battery, the quantity of sulphuricacid is much lower than the quantity of active matter likely to beoxidized or reduced. Their ratio is in the order of 15%. To this acid,present on site, additions are made during discharge brought bydiffusion from outside the plates, which are more significant the slowerthe discharge. The coefficients of use of the active matter are highestin the vicinity of the surface of the plates. They may reach 60 to 70%,to be compared with the average value of 25 to 30% for the entireelectrode.

Document FR-A-2 438 346 [I] and the publication by J. Alzieu et al., atthe Fifth International Electric Vehicle Symposium, Philadelphia, Oct.2-5, 1978 [2], describe lead-acid batteries with a long lifetime. Thesebatteries have a positive electrode, a negative electrode, anelectrolyte formed of sulphuric acid, a set of separator elementsarranged between the positive electrode and the negative electrode andmeans for applying pressure to the whole assembly. It is indicated thatwith the application of pressure it is possible in particular toincrease the lifetime of these lead-acid batteries.

The document J. Electrochem. Soc., vol 130, No. 11, 1983, pages2144-2149 [3] illustrates a lead-acid battery which uses an activematerial for the positive electrode having a density of 3.9 g/cm³ ₁, andan electrolyte made of sulphuric acid having a density of 1.28 at 20° C.With the use of such materials it is possible to restrict changes in thestructure and physical properties of the active material of the positiveelectrode, for the purpose of improving its lifetime. In this document,pressure is also applied to the assembly of electrodes, by which meansit is also possible to limit electrode structural changes.

The improvement brought by placing the electrodes under stress is ofinterest, but the energy-to-weight ratio of lead-acid batteries stillremains insufficient compared with the performance it is desired toobtain.

Other improvements in lead-acid batteries have been considered by H.Ozgun et al., Journal of Power Sources, 52, 1994, pages 159-171 [4].These improvements concern variation in the density of the activematerial of the electrodes. In this latter case, the authors recommendincreasing the density of the active material in order to increase thebattery's charging/discharging cycle capacity.

P. W. Appel and D. B. Edwards in Journal of Power Sources, 55, 1995,pages 81-85 [5] endeavoured to improve the performance of the lead-acidbattery by improving the conductivity of the active material throughincorporation of conductor particles, but they did not succeed infinding particles that were able to withstand the particularly corrosivemedium of a positive electrode in a lead-acid battery.

Application of pressure to the electrode should bring about animprovement in the coefficient of use of the reagents inside theelectrodes by using electrodes in the form of thin plates. It can beunderstood that a thin plate, that is to say in which every activematerial element is near a surface delimiting the plate, can haveimproved performance; the overall coefficient of use should be expectedto be 60 to 70%.

With compression it is possible to remedy the special fragile nature ofthese thin plates. After considering a move in this direction, currentlyone of the lines of research adopted by the ALABA Advanced Lead AcidBattery Consortium, this approach has been abandoned since unsuspectedexperimental results have opened up new prospects.

The subject of the present invention is precisely a lead-acid battery ofthe type described in documents [1] to [3], which has an improvedenergy-to-weight ratio due to arrangements allowing an increase in thecoefficient of use of the active materials of the electrodes byachieving a better distribution of the reagents within the electrodes.

DISCLOSURE OF THE INVENTION

The subject of the present invention is a leadacid battery containing:

a positive electrode containing lead oxide as active material,

a negative electrode containing lead sponge as active material,

an electrolyte formed of a solution of sulphuric acid,

a separator element between the positive electrode and the negativeelectrode, and

means for applying a stress to the entire assembly perpendicular to theplane of the electrodes, in which, in the charged state, the quantity ofsulphuric acid in the positive electrode represents at least 0.20 moleof H₂SO₄ per mole of active material of the positive electrode, and/orthe quantity of sulphuric acid in the negative electrode represents atleast 0.20 mole of H₂SO₄ per mole of active material in the negativeelectrode.

This quantity of sulphuric acid in the positive or negative electrodemay, for example, represent from 0.20 to 1 mole, or from 0.20 to 0.70mole of H₂SO₄ per mole of active material in the electrode.

Preferably, according to the invention, in the battery in the chargedstate, the quantity of sulphuric acid in the positive electroderepresents at least 0.25 and even better 0.40 mole of H₂SO₄ per mole ofactive material in the positive electrode, and/or the quantity ofsulphuric acid in the negative electrode represents at least 0.25 andeven better 0.40 mole of H₂SO₄ per mole of active material in thenegative electrode.

In the lead-acid battery of the invention, this improved distribution ofreagents within the electrodes can be obtained by adopting one or moreof the following arrangements, compared with the prior art:

1) modifying the structure of the positive electrode in order toincrease the quantity of sulphuric acid in the positive electrode,

2) modifying the structure of the negative electrode in order toincrease the quantity of sulphuric acid in the negative electrode, and

3) increasing the H₂SO₄ concentration of the electrolyte.

According to the invention, it is possible to use simultaneously two orthree of these modifications to adjust sulphuric acid quantities in thepositive electrode and/or in the negative electrode to desired values.

For, according to the invention, it has been discovered that thequalities of sturdiness of lead-acid batteries achieved by electrodecompression, by applying a compression stress to the electrodes in theorder of 0.01 to 0.3 MPa, made it possible to apply other arrangementsable to improve the coefficient of use of the active material in thepositive electrode, whereas such arrangements would have been hazardousin a conventional battery structure.

These arrangements particularly concern:

increasing the H₂SO₄ concentration of the electrolyte,

increasing the porosity of the electrodes, and

including porous elements in these electrodes.

Therefore, according to a first embodiment of the invention, theporosity of the positive and/or of the negative electrode is modified.In this case, an electrode active material is used having an apparentdensity in the dry, charged state of 2.8 to 3.2 g/cm³, preferably from3.0 to 3.2 g/cm³.

By increasing the porosity of the active material of the positiveelectrode and/or of the negative electrode, it is possible to increasethe quantity of sulphuric acid per mole of active material, andtherefore to promote exchanges between the active material of theelectrode and the electrolyte.

To obtain an electrode active material with increased porosity, it ispossible to proceed in the following manner.

As a general rule, the active material of the electrodes is obtainedfrom a paste made of lead oxide, water and sulphuric acid by pasting agrid which serves to collect the current, followed by drying, then bymaturing for 48 hours in a saturated steam atmosphere. By subsequentlyapplying an electric current to the electrodes, lead dioxide PbO₂ isformed which serves as active material for the positive electrode andspongelike metallic lead is formed which serves as the active materialfor the negative electrode. In this production, the water and acidcontent of the lead oxide paste regulates the porosity of the activematerial subsequently obtained. According to the invention, all that isneeded therefore is to adjust the water and acid content of the paste toobtain an active material having an apparent density lying within therange described above.

A further means of increasing the porosity of the active material of anelectrode of the prior art, is to submit this electrode which contains amaterial generally having an apparent density in the dry, charged stateof 3.3 to 3.6 g/cm³, to electric treatment consisting of at least onedeep discharge followed by recharging.

This may be carried out by placing an element, made of conventionalelectrodes, in short-circuit at the end of discharge, for 48 hours forexample. The effect of this deep discharge is to swell up the positiveelectrode and/or negative electrode, and hence to increase its porosity.However, after 150 to 200 charging/discharging cycles under normaloperating conditions, the porosity of the electrode may have decreasedand returned to its initial value. In this case, the increased porosityvalue can be restored by causing the electrode to re-undergo at leastone deep discharging cycle.

According to a second embodiment of the invention, the structure of thepositive electrode and/or of the negative electrode is modified byadding inert porous particles, able to charge themselves withelectrolyte, in the active material of these electrodes. These porousparticles may be microporous fragments of inert material such aspolyethylene, polypropylene or any other polymer resistant to theelectrolyte. The presence of electrolyte in these porous particles alsomakes it possible to increase the quantity of electrolyte per mole ofactive material in the electrode. These porous particles may be added tothe lead oxide paste used to line the grids or current collectors.

It is possible, in particular, to add a quantity of porous particles inthe active material in such manner that they represent 5 to 80%,preferably 10 to 50%, of the final volume of the electrode material. Inthis way, the structure of the electrode active material is modified byincreasing its porosity and its electrolyte content.

According to a third embodiment of the invention, the quantity ofelectrolyte is adjusted per mole of active material of the electrodes byincreasing the density of the electrolyte. In this case, it is possibleto use a solution of H₂SO₄ having a density of at least 1.30, forexample from 1.30 to 1.50, and preferably from 1.32 to 1.40.

Advantageously, this third embodiment of the invention is combined withone of the two preceding embodiments.

Other characteristics and advantages of the invention will be betterunderstood on reading the following description of examples ofembodiment, which are evidently given for illustration purposes only andare not restrictive, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a vertical section diagram of a lead-acid battery in which theimprovements of the invention can be used.

FIG. 2 is a perspective view of the assembly of the positive andnegative electrodes in FIG. 1 with the application of pressure to theassembly.

FIG. 3 is a perspective view of an embodiment of the positive electrodeof the FIG. 1 arranged in a microporous separator.

FIG. 4 illustrates the changes in the average pore radius of the activematerial in positive battery electrodes in relation to the number ofcycles, in the case in which the electrode is subjected to a stress(curve 1) and when there is no stress (curve 2).

FIG. 5 shows the changes in capacity (in A.h.) of a battery under stressin relation to the number of cycles when several deep discharges of theelectrodes are made between the 530^(th) and 750^(th) cycles.

FIG. 6 illustrates the improvement obtained by increasing the ratiobetween the quantity of electrolyte and the quantity of active material,by forced circulation of electrolyte.

In FIG. 1, a lead-acid battery is shown comprising a positive electrode1 in the form of a plate and a negative electrode 3 in the form of aplate, both plates separated from one another by an assembly comprisinga separator element 5 that is corrugated and perforated, made inpolyvinyl chloride PVC for example, and an element 7 such as fibrousnetting made in glass fibre for example. The positive and negativeelectrodes are bathed in an electrolyte 9.

In this battery, the positive electrode is formed of lead oxide PbO₂which covers an alveolar lead sheet or grid.

Negative electrode 4 is similarly formed from lead sponge.

Preferably, according to the invention, as shown in FIG. 1, the positiveelectrode 1 is also separated from electrolyte 9 by a microporouselement 11 which may formed of so-called Darak® paper made up of paper,glass wool and phenol resin.

In FIG. 2, the battery of FIG. 1 is shown, completed with the means forapplying a stress to the assembly perpendicular to the plane ofelectrodes 1 and 3. In this case, the assembly 12 is arranged betweentwo metal disks 13 and 15 which are connected to one another by a set ofthreaded rods 17 fitted with bolts 19 and tared springs 20.

It is therefore possible to apply to the assembly of electrodes 1 and 3a stress in the region of 0.01 to 0.3 MPa, preferably a stress in theorder of 0.1 MPa.

FIG. 3 shows a perspective view of positive electrode 1 surrounded bythe microporous separator 11. In this case, this separator entirelysurrounds the electrode and is formed of two microporous separatorswelded on their periphery, for example with a line of thermosettingresin 12.

It is specified that in the lead-acid battery of the invention, theseparator assembly formed by the separator element 5 and element 7 maybe replaced by a single element formed of a glass fibre mat.

According to the invention, the performance of the battery described inFIGS. 1 to 3 is improved by using positive and/or negative electrodeshaving structures which lead to a higher quantity of electrolyte permole of active material.

It was found that the application of a stress to the electrodes enabledthe porosity of the electrode to be stabilized during cycling.

FIG. 4, curve 1, shows the changes in the average pore radius (in μm) ofthe active matter of a positive electrode under stress in relation tothe number of cycles made.

In this figure, it can be seen that the average pore radius of theelectrode increases firstly and then stabilizes after a slight decreaseuntil the end of the lifetime of the battery (3000 cycles).

By way of comparison, this figure also shows, in curve 2, the changes inthe average pore radius of a similar electrode used in a battery withoutapplication of a stress. In this case, the average pore radius increasesover the entire cycling until the end of the lifetime (approximately 800cycles).

According to the invention, by means of the electrode porositystabilizing effect due to stress, it is possible to considerablyincrease the initial porosity of the electrode without harming thelifetime of this electrode.

On the other hand, if no stress is applied, it is known that anelectrode whose initial porosity is more than 10% greater than that ofconventional electrodes, has a lifetime which is reduced in unacceptablemanner.

According to the invention, it is therefore possible to improve theperformance of the lead-acid battery under stress as described above, byusing materials having higher porosity as active materials for thepositive and/or negative electrodes.

It is recalled that positive electrodes are generally prepared from alead oxide paste which covers the lead alveolar sheets or grids used assupport. After pasting these grids, they are left to dry and mature for48 hours in a steam-saturated atmosphere. The active material PbO₂ isthen formed by the application of an electric current. The activematerial of the negative electrodes is prepared in the same manner, butwhen the electric current is applied the lead oxide paste is reduced tolead sponge.

According to the invention, higher porosity of these active materials isobtained using a higher water and acid content in the paste.

It is also possible to obtain higher porosity of the active material ofthe electrode by carrying out a deep discharge of the electrode preparedfrom a lead oxide paste of conventional composition.

FIG. 5 shows the capacity (in A.h.), in relation to number of cycles, ofa battery under stress subjected to several deep discharges.

The cycling used was as follows:

discharge under constant current (11 A) for 3 h, and

charging under constant current in two levels:

level one: 6.65 A for 5 h, and

level two: 2 A for 4 h.

The capacities are measured during so-called control cycles, identicalto the preceding ones except that for charging the second level under 2Ais extended by 2 h, and discharging under 11 A is extended until thevoltage at the battery terminals reaches 1.70 V.

As shown in FIG. 5, the capacity increases from the 1^(st) to the65^(th) cycle, and then decreases to stabilize at around 39 A.h.

At the 530^(th) cycle, a deep discharge is carried out followed byrecharging, and it is found that the capacity of the battery increasesup to 45 A.h., i.e. by 15%.

From the 530^(th) to the 750^(th) cycle, various deep discharges aremade which are of the two following types:

discharge under reduced current until the voltage falls to less than 1 Vper element of the battery, which may go as far as short-circuitingmaintained for 60 h,

succession of cycles comprising complete discharge, whereas up until the530^(th) cycle the discharge was interrupted after 3 hours.

These two types of discharge were experimented separately and incombined manner. Periods of return to initial cycling made it possibleto verify that the increase in capacity observed was not of a fleetingnature. In this manner, under a control discharge current of 11 A, acapacity of 60 A.h. is reached whereas the nominal capacity for thiscurrent is 39 A.h., i.e. an overall gain of more than 50%.

On and after the 750^(th) cycle, a further cycling as defined above isconducted. The objective is to analyze, in the absence of a deepdischarge, the durable or non-durable character of the increase incapacity. After 250 cycles, that is to say at the 1000^(th) cycle, thecapacity measured is always greater than the nominal capacity. Thedecrease appears to be linear. The slope is 5.5 A.h. per 100 cycles.

This result is fully surprising since normally the battery would beexpected to be irretrievably deteriorated by a deep discharge obtainedby extended short-circuiting.

All the more surprising is the fact that after further deep discharges,an additional increase is obtained in battery capacity which may exceedan overall gain of 50%.

This phenomenon could be attributed either to the improved conductivityof the active material, or to the increase in porosity. Experiments onconductivity have led to eliminating the hypothesis of reorganisation ofthe conductor framework which ensures charge collection within theactive matter, and to preferring the assumption of a considerableincrease in porosity of this active material. Subsequent researchconducted confirmed this viewpoint.

It is known that during discharge of the positive electrode, the leadoxide is converted into lead sulphate according to the followingreaction diagram:

PbO₂+H₂+H₂SO₄→PbSO₄+2H₂O

During discharge of the negative electrode, the lead is converted intolead sulphate according to the following reaction diagram:

Pb+SO₄→PbSO₄

However, the lead sulphate produced has a higher volume than theoriginal lead oxide or lead.

If a deep discharge is carried out, for example by short-circuiting theelectrode at the end of discharging for 48 hours, and if the electrodeis subsequently charged, the active material which converts to PbO₂ orPb does not resume its initial volume. Therefore an increase in porosityis obtained which permits improvement in battery performance.

This increase is not long-lasting since after a period of 150 to 200cycles, the active material of the electrode may resume its initialvolume. However, it is possible to restore this higher porosity by againsubmitting the electrode to a deep discharge as indicated above.

A further possible way of increasing the porosity of the active materialin each of the electrodes is to include porous elements in the material,for example porous particles incorporated in the lead oxide paste usedto make the electrode.

Porous particles which may be used can be polyethylene particles whosesmallest size is no more than 0.5 mm, preferably approximately 0.2 mm.

In the electrode(s), it is possible to use a proportion of porousparticles representing from 2 to 50%, preferably from 5 to 30%, of thefinal volume of the electrode material. These particles may have aporosity of 60 to 70% by volume, which makes it possible to include anadditional quantity of electrolyte in the active material.

According to the invention, it is also possible to improve theperformance of the battery described in FIGS. 1 to 3 by using anelectrolyte made up of a more concentrated solution of sulphuric acidhaving a density of at least 1.30, for example of 1.30 to 1.50, andpreferably from 1.32 to 1.40.

The use of such high densities of sulphuric acid does not harm thelifetime of the lead-acid batteries in which, according to theinvention, a stress is applied to the positive and negative electrodes.

By way of example, a lead-acid battery conforming to reference [1]having a capacity of 26 A.h. with an electrolyte having a density of1.28 will have this capacity increased to 30 A.h. with an electrolytehaving a density of 1.32, and to 45 A.h. with an electrolyte having adensity of 1.32 after six deep discharges. FIG. 6 illustrates thebeneficial effect of an increased quantity of electrolyte per mole ofactive material. This FIG. 6 shows the changes in the rate of conversion(by %) of the active material of the positive electrode in relation tothe discharge current (in amperes) for a lead-acid battery conforming toreference [1] (curve 41) and for a lead-acid battery of the same type(curve 43) in which the exchanges between the electrolyte and the activematerial have been promoted by producing forced circulation ofelectrolyte through the active material at a rate of 2 μm/s, whichcorresponds to an increased quantity of electrolyte per mole of activematerial.

In this figure, it can be clearly seen that electrolyte deficit is thelimiting factor in respect of the rate of conversion of the activematerial in the positive electrode. For a discharge current of 1.25A,this conversion level is increased by 80% with forced circulation of theelectrolyte, which confirms the advantage of the invention.

It is specified that in all the embodiments of the invention, theelectrolyte used, which is formed of more or less dilute sulphuric acid,may be in the form of a gel, for example by means of SiO₂, or in a formabsorbed in an appropriate material which may be a mat of micro-glassfibres acting as separator, or a microporous separator such as thosedenoted 5 and 11 in FIG. 1.

Although the above-described examples relate to a single pole battery,the invention evidently also applies to bipolar batteries.

In this case, the positive electrode and the negative electrode of thebattery are associated together to form bipolar plates comprising, onone surface, the active material of the positive electrode and on theother surface the active material of the negative electrode.

If, with these bipolar plates, the previously described arrangements areused allowing an increase in the coefficient of use of the activematerials of the electrodes through improved distribution of thereagents within the electrodes, it is possible not only to obtain animprovement in the power-to-weight ratio due to the bipolar structurebut also an improvement in the energy-to-weight ratio.

Any bipolar wall leads to a substantial improvement in power-to-weightratio for short demands of power. On the other hand, if no otherarrangement is present, the energy-to-weight ratio is reduced,especially with 1 to 2-hour discharge times characteristic of electricvehicles. This is due to the fact that the electrodes, placed up againstthe bipolar wall, only offer one surface to the electrolyte whichreduces by a factor of 2 the supply of reactive species from theelectrolyte outside the electrodes.

The combination of improved coefficient of use of active material andthe application of a bipolar structure would bring gains both in respectof power-to-weight ratio and in respect of energy-to-weight ratio.

What is claimed is:
 1. Lead-acid battery comprising: a positiveelectrode (1) containing lead oxide as active material, a negativeelectrode (3) containing lead sponge as active material, an electrolyte(9) formed of a solution of sulphuric acid, a separator element (5, 7)between the positive electrode and the negative electrode, and means forapplying a stress to the assembly perpendicular to the plane of theelectrodes, in which, in the charged state, the quantity of sulphuricacid in the positive electrode represents at least 0.20 mole of H₂SO₄per mole of active material in the positive electrode, and/or thequantity of sulphuric acid in the negative electrode represents at least0.20 mole of H₂SO₄ per mole of active material in the negativeelectrode, and in which the stress applied to the electrodes is 0.01 to0.3 Mpa.
 2. Battery according to claim 1, in which, in the chargedstate, the quantity of sulphuric acid in the positive electroderepresents at least 0.25 mole of H₂SO₄ per mole of active material inthe positive electrode, and/or the quantity of sulphuric acid in thenegative electrode represents at least 0.25 mole of H₂SO₄ per mole ofactive material in the negative electrode.
 3. Battery according to claim1, in which the active material of the positive electrode has anapparent density in the dry, charged state of 2.8 to 3.2 g/cm³. 4.Battery according to claim 1, in which the active material of thenegative electrode has an apparent density in the dry, charged state of2.8 to 3.2 g/cm³.
 5. Battery according to claim 1, in which a positiveelectrode is used whose active material has an apparent density in thedry, charged state of 3.3 to 3.6 g/cm³ and which is caused to undergo atleast one deep discharge followed by recharging to bring its apparentdensity in the dry, charged state to a value of 2.8 to 3.2 g/cm³. 6.Battery according to claim 1, in which a negative electrode is usedwhose active material has an apparent density in the dry, charged stateof 3.3 to 3.6 g/cm³ and which is caused to undergo at least one deepdischarge followed by recharging to bring its apparent density in thedry, charged state to a value of 2.8 to 3.2 g/cm³.
 7. Lead-acid batteryaccording to claim 1, in which the active material of the positiveelectrode and/or the active material of the negative electrode alsocontains inert porous particles able to charge themselves withelectrolyte.
 8. Battery according to claim 7, in which the porousparticles are in polyethylene.
 9. Battery according to claim 7, in whichthe quantity of porous particles represents 5 to 80% of the final volumeof electrode material.
 10. Battery according to claim 7, in which thequantity of porous particles represents between 10 and 50% of the finalvolume of electrode material.
 11. Battery according to claim 1, in whicha solution of sulphuric acid is used as electrolyte having a density ofat least 1.30.
 12. Battery according to claim 11, in which the densityof the sulphuric acid solution ranges from 1.30 to 1.50.
 13. Batteryaccording to claim 1, in which the positive electrode and the negativeelectrode are associated together to form bipolar plates comprising, onone surface, the active material of the positive electrode, and on theother surface the active material of the negative electrode.
 14. Batteryaccording to claim 11, in which the density of the sulphuric acidsolution ranges from 1.32 to 1.40.